The present invention relates to an electophotographic copier, printer, facsimile apparatus or similar image forming apparatus of the type having a paper transport path which includes an intermediate tray for temporarily stacking paper sheets each carrying an image on one surface thereof. More particularly, the present invention is concerned with a device for controlling the stacking of such paper sheets on the intermediate tray.
An electrophotographic copier operable in a combination copy mode and a two-sided copy mode is extensively used today. In the combination copy mode, images are reproduced one above another on the same surface of a paper sheet while, in the two-sided copy mode, images are reproduced on both surfaces of a paper sheet. With this type of copier, it is necessary to turn over a paper sheet once on a paper transport path which is defined in the copier. The turn-over is often implemented by a switchback path which branches off a usual paper transport path. Specifically, a paper sheet once driven into the switchback path is switched back so that the paper sheet may then be driven out of the switchback path with its trailing edge standing first. For example, assume a copier whose paper transport path is configured such that a paper sheet driven along a usual transport path is fed to an intermediate tray without being turned over. In such a copier, the switchback path is defined upstream or downstream of the intermediate tray and, in a two-sided copy mode, a paper sheet is once fed into the switchback path and then switched back to leave the switchback path with its trailing end in the lead. Concerning a copier of the type having a paper transport path which causes a paper sheet to be fed to an intermediate tray and then refed from the tray with its trailing edge standing first, the switchback path is defined downstream or upstream of the intermediate tray so that in a combination copy mode a paper sheet may be fed into the switchback once and then switched back.
A problem often encountered with a copier of the type transporting a paper sheet to an intermediate tray and then refeeding it out of the tray is that a paper sheet undergone an image fixing step is curled. A curl would prevent paper sheets from being neatly stacked on the intermediate tray. To eliminate this problem, a paper guide member may be used which presses the upper surface of the uppermost paper sheet relatively lightly, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 57-72525 by way of example.
However, in a copier having a long paper transport path such as a switchback path and implementing the combination copy mode or the two-sided copy mode by turning over a paper sheet at a position upstream or downstream of an intermediate tray as stated above, it is necessary for the switchback path to have a length which is great enough to accommodate the entire length of the largest size of paper sheets usable with the copier. This not only increases the overall dimensions of the copier but also increases the length of the transport path and thereby the copying time. Such a long switchback path needs a substantial number of structural parts and therefore a substantial cost and is complicated to degrade the reliability of operation. Further, since the prior art paper guide member for eliminating the curl problem previously discussed exerts only a small force which lightly presses the upper surface of the uppermost paper on the intermediate tray, a paper sheet reaching the intermediate tray is apt to dislocate a paper sheet previously stacked on the tray due to friction between the paper sheets, elasticity of the paper sheets, etc.